I. Batrachia.
Animals which compose this class have long been confounded with reptiles, from which they differ in one fundamental peculiarity in their organization. At their birth they respire by means of gills, and consequently resemble fishes. In a physiological point of view, at a certain time in their lives, these animals are fishes in form as well as in their habits and organization. As age progresses, they undergo permanent metamorphosis—they acquire lungs, and thenceforth an aërial respiration. It is, then, easy to understand that these animals hold a doubtful rank, as they have long done, amongst Reptiles, which are animals with an aërial respiration; they ought to form a separate class of Vertebrates.[8]
Batrachians establish a transitional link between Fishes and Reptiles—they are, as it were, a bond of union between those two groups of animals. In the adult state Batrachians are cold-blooded animals with incomplete circulation, inactive respiration, and the skin is bare. In the introductory section to this chapter we have given the general characteristics which belong to them. The Frogs—Tree Frogs, Toads, Surinam Toads, Salamanders, and Newts—are the representatives of the principal families of Batrachians of which we propose giving the history.
The Frogs, Rana, have been irreparably injured by their resemblance to the Toads. This circumstance has given rise to an unfavourable prejudice against these innocent little Batrachians. Had the Toad not existed, the Frog would appear to us as an animal of a curious form, and would interest us by the phenomena of transformation which it undergoes in the different epochs of its development. We should see in it a useful inoffensive animal of slender form, with delicate and supple limbs, arrayed in that green colour which is so pleasant to the eye, and which mingles so harmoniously with the carpeting of our fields.
Fig. 4.—The Edible Frog (Rana esculenta).
The body of the Edible Frog, Rana esculenta ([Fig. 4]), sometimes attains from six to eight inches in length, from the extremity of the muzzle to the end of the hind feet. The muzzle terminates in a point; the eyes are large, brilliant, and surrounded with a circle of gold colour. The mouth is large; the body, which is contracted behind, presents a tubercular and rugged back. It is of a more or less decided green colour on the upper, and whitish on the under parts. These two colours, which harmonize well, are relieved by three yellow lines, which extend the whole length of the back, and by scattered black marbling. It is, therefore, much to be regretted that prejudice should cause some at least of us to turn away from this pretty little hopping animal, when met with in the country; with its slight dimensions, quick movements, and graceful attitudes. For ourselves, we cannot see the banks of our streams embellished by the colours and animated with the gambols of these little animals without pleasure. Why should we not follow with our eyes their movements in our ponds, where they enliven the solitude without disturbing its tranquillity. Frogs often leave the water, not only to seek their nourishment, but to warm themselves in the sun. When they repose thus, with the head lifted up, the body raised in front and supported upon the hind feet, the attitude is more that of an animal of higher organization than that of a mean and humble Batrachian. Frogs feed on larvæ, aquatic insects, worms, and small mollusks. They choose their prey from living and moving creatures; for they set a watch, and when they perceive it, they spring on it with great vivacity. A large Indian species (R. tigrina) has been seen to prey occasionally upon young Sparrows. Far from being dumb, like many oviparous quadrupeds, Frogs have the gift of voice. The females only make a peculiar low growl, produced by the air which vibrates in the interior of two vocal pouches placed on the sides of the neck; but the cry of the male is sonorous, and heard at a great distance: it is a croak which the Greek poet, Aristophanes, endeavoured to imitate by the inharmonic consonants, brekekurkoax, coax! It is principally during rain, or in the evenings and mornings of hot days, that Frogs utter their confused sounds. Their chanting in monotonous chorus makes this sad melody very tiresome. Under the feudal system, during the "good old times" of the middle ages, which some people would like to bring back again, the country seats of many of the nobility and country squires were surrounded by ditches half full of water, all inhabited by a population of croaking Frogs. Vassals and villains were ordered to beat the water in these ditches morning and evening in order to keep off the Frogs which troubled the sleep of the lords and masters of the houses. Independent of the resounding and prolonged cries of which we have spoken, at certain times the male Frog calls the female in a dull voice, so plaintive that the Romans described it by the words "ololo," or "ololygo." "Truly," says Lacépède, "the accent of love is always mingled with some sweetness."
When autumn arrives Frogs cease from their habitual voracity, and no longer eat. To protect themselves from the cold, they bury themselves deeply in the mud: troops of them joining together in the same place. Thus hidden, they pass the winter in a state of torpor; sometimes the cold freezes their bodies without killing them. This state of torpor gives way in the first days of spring. During the month of March, Frogs begin to awake and to move themselves; this is their breeding season. Their race is so prolific that a female can produce from six to twelve hundred eggs annually. These eggs are globular, and are in form a glutinous and transparent spheroid, at the centre of which is a little blackish globule; the eggs float, and form like chaplets on the surface of the water.
Fig. 5.—Development of the Tadpole.
1. Egg of the Frog. 2. The Egg fecundated, and surrounded by its visicule. 3. First state of the Tadpole. 4. Appearance of the breathing gills. 5. Their development. 6. Formation of the hind feet. 7. Formation of the fore feet, and decay of the gills. 8. Development of the lungs, and reduction of the tail. 9. The perfect Frog.
All who have observed the small ponds and ditches in the country at this season, will have seen these light and elegant crafts swimming on the surface of the water. After a few days, more or less according to the temperature, the little black spot which is the embryo of the egg, and which has developed itself in the interior of the glairy mass which envelops it, disengages itself and shoots forth into the water: this is the tadpole of the Frog.
The body of the tadpole is oval in shape, and terminates in a long flat tail, which forms a true fin; on each side of the neck are two large gills, in shape like a plume of feathers; the tadpole has no legs. These gills soon begin to wither, without aquatic respiration ceasing, however; for, besides these, the tadpole possesses interior gills like fishes. Soon after, the legs begin to show themselves, the hind legs appearing first; they acquire a considerable length before the fore feet begin to show themselves. These develop themselves under the skin, which they presently pierce through. When the legs have appeared, the tail begins to fade, and, little by little, withers away, until in the perfect animal it entirely disappears. About the same time the lungs become developed, and assume their functions. In [Fig. 5] may be traced the successive phases of its transformation from the egg to the tadpole, till we finally reach the perfect Batrachian. Through these admirable modifications we see the Fish, little by little, become a Batrachian. In order to follow this strange metamorphosis, it suffices to gather some Frog's eggs, and to place them with some aquatic herbs in an aquarium, or in a globe with Gold and Silver Fish; it there constitutes a most interesting spectacle, and we advise our readers to give themselves this instructive and easy lesson in natural history.
At present, there exist two species of Frog in Europe: the Green or Edible Frog, and the Common Frog. The Green Frog is that which we have described, and of which we have given a representation in [Fig. 4]. They are found in running streams and stagnant waters. It is this species to which La Fontaine alludes in one of his fables. Common Frogs are smaller than the preceding: they inhabit damp places in fields and vineyards, and only return to the water to breed or to winter.
The flesh of the Edible Frog is very tender, white, and delicate. As an article of food, it is lightly esteemed by some, but undeservedly so. Prepared in the same manner, Green Frogs closely resemble very young fowls in taste. In almost all parts of France Frogs are disdained as articles of food; it is only in the south that a taste for them is openly avowed, and there Frogs are sought for and brought to market. Therefore, I never could comprehend how the notion popular in England, when it is wished to express contempt for Frenchmen, should be to call them Frog-eaters. It is a reproach which might be addressed to Provençals and Languedocians like the author of this work, but not at all to the majority of Frenchmen.
Fig. 6.—Green Tree Frog (Hyla).
The Green Tree Frog is easily distinguished by having little plates under its toes. These organs are a species of sucker, by means of which the animal is enabled, like the house-fly, to cling strongly to any surface, however smooth and polished it may be. The smoothest branch, even the lower surface of a leaf, forms a sufficient hold and support to these delicate organs.
The upper part of the body is of a beautiful green, the lower part, where little tuberculi are visible, is white. A yellow line, lightly bordered with violet, extends on each side of the head and back, from the muzzle to the hind legs. A similar line runs from the jaw to the front legs. The head is short, the mouth round, and the eyes raised. Much smaller than the ordinary Frog, they are far more graceful. During the summer they live upon the leaves of trees in damp woods, and pass the winter at the bottom of some pond, which they do not leave till the month of May, after having deposited their eggs. They feed on small insects, worms, and mollusks; and in order to catch them, they will remain in the same place an entire day. During the glare of the sun, they remain hidden amongst the leaves; but when twilight approaches, they move about and climb up the trees. We must repeat of these Green Tree Frogs what we have already said of Frogs. Get rid of all prejudice towards their kind, and then you will examine with pleasure their lively colours, which harmonize so well with the green leaves; remark their tricks and ambuscades; follow them in their little hunting excursions; see them suspended upside down upon the leaves in a manner which appears marvellous to those who are not aware of the organs which have been given to enable them to attach themselves to the smoothest bodies: and it will give as much pleasure as can be derived from the consideration of the plumage, habits, and flight of birds. The croak of the Green Tree Frogs is like that of other Frogs, although less sharp and sometimes stronger in the males; it can be pretty well translated by the syllables caraccarac, pronounced from the throat. This cry is principally heard in the morning and evening; then, when one Frog begins to utter its croak, all the others imitate it. In the quiet night the voice of a troop of these little Batrachians sometimes reaches to an enormous distance.
Toads, Bufo, are squat and disagreeable in shape: it is difficult to comprehend why nature, which has bestowed elegance and a kind of grace upon Frogs and Tree Frogs, has stamped the Toad with so repulsive a form. These much despised beings occupy a large place in the order of nature: they are distributed with profusion, but one cannot say exactly to what end; their movements are heavy and sluggish. In colour they are usually of a livid grey, spotted with brown and yellow, and disfigured by a number of pustules or warts. A thick and hard skin covers a flat back; its large belly always appears to be swollen; the head a little broader than the rest of its body; the mouth and the eyes are large and prominent. It lives chiefly at the bottom of ditches, especially those where stagnant and corrupt water has lain a long time. It is found in dung heaps, caves, and in dark and damp parts of woods. One has often been disagreeably surprised on raising some great stone to discover a Toad cowering against the earth, frightful to see, but timorous, seeking to avoid the notice of strangers. It is in these different obscure and sometimes fœtid places of refuge that the Toad shuts itself up during the day; going out in the evening, when our common species moves by slight hops; whilst another, the Natterjack Toad, Bufo calamita, only crawls, though somewhat fastly. When seized, it voids into the hand a quantity of limpid water imbibed through the pores of its skin; but if more irritated, a milky and venemous humour issues from the glands of its back.
Fig. 7.—The Common Toad (Bufo vulgaris).
One peculiarity of its structure offers a defence from outward attacks. Its very extensible skin adheres feebly to the muscles, and at the will of the animal a large quantity of air enters between this integument and the flesh, which distends the body, and fills the vacant space with an elastic bed of gas, by means of which it is less sensible to blows. Toads feed upon insects, worms, and small mollusks. In fine evenings, at certain seasons especially, they may be heard uttering a plaintive monotonous sound. They assemble in ponds, or even in simple puddles of water, where they breed and deposit their eggs. When hatched, the young Toads go through the same metamorphosis as do the tadpoles of the Frogs.
Their simple lives, though very inactive, are nevertheless very enduring; they respire little, are susceptible of hibernation, and can remain for a considerable time shut up in a very confined place.
It is proper, however, to caution the reader against believing all that has been written about the longevity of Toads. Neither must implicit faith be given to the discovery of the living animal ([Fig. 7]) in the centre of stones. "That Toads, Frogs, and Newts, occasionally issue from stones broken in a quarry or in sinking wells, and even from coal-strata at the bottom of a mine," is true enough; but, as Dr. Buckland observes, "the evidence is never perfect to show that these Amphibians were entirely enclosed in a solid rock; no examination is made until the creature is discovered by the breaking of the mass in which it was contained, and then it is too late to ascertain whether there was any hole or crevice by which it might have entered." These considerations led Dr. Buckland to undertake certain experiments to test the fact. He caused blocks of coarse oölitic limestone and sandstone to be prepared with cells of various sizes, in which he enclosed Toads of different ages. The small Toads enclosed in the sandstone were found to die at the end of thirteen months; the same fate befell the larger ones during the second year: they were watched through the glass covers of their cells, and were never seen in a state of torpor, but at each successive examination they had become more meagre, until at last they were found dead. This was probably too severe a test for the poor creatures, the glass cover implying a degree of hardness and dryness not natural to half amphibious Toads. Moreover, it is certain that both Toads and Frogs possess a singular facility for concealing themselves in the smallest crevices of the earth, or in the smallest anfractuosities of stones placed in dark places.
This animal, so repulsive in form, has been furnished by nature with a most efficient defensive armature; namely, an acrid secretion which will be described farther on. It is a bad leaper, an obscure and solitary creature, which shuns the sight of man, as if it comprehended the blot it is on the fair face of creation. It is, nevertheless, susceptible of education, and has occasionally been tamed; but these occasions have been rare. Pennant, the zoologist, relates some curious details respecting a poor Toad, which took refuge under the staircase of a house. It was accustomed to come every evening into a dining-room near to the place of its retreat. When it saw the light it allowed itself to be placed on a table, where they gave it worms, wood-lice, and various insects. As no attempt was made to injure it, there were no signs of irritation when it was touched, and it soon became, from its gentleness (the gentleness of a Toad!), the object of general curiosity; even ladies stopped to see this strange animal. The poor Batrachian lived thus for six and thirty years; and it would probably have lived much longer had not a Crow, tamed, and, like it, a guest in the house, attacked him at the entrance of his hole, and put out one of his eyes. From that time he languished, and died at the end of a year.
Fig. 8.—Surinam Toad (Pipa monstrosa).
Nearly allied to the Toads, Bufo, the Surinam Toad, Pipa, holds its place. Its physiognomy is at once hideous and peculiarly odd: the head is flat and triangular, a very short neck separates it from the trunk, which is itself depressed and flattened. Its eyes are extremely small, of an olive, more or less bright, dashed with small reddish spots. It has no tongue. There is only one species of Pipa, viz. the American Pipa ([Fig. 8]), which inhabits Guiana and several provinces of Brazil. The most remarkable feature in this Batrachian is its manner of reproduction. It is oviparous, and when the female has laid her eggs, the male takes them, and piles upon the back of his companion these, his hopes of posterity. The female, bearing the fertilized eggs upon her back, reaches the marshes, and there immerses herself; but the skin of the back which supports the eggs soon becomes inflamed, erysipelatous inflammation follows, causing an irritation, produced by the presence of eggs, which are then absorbed into the skin, and disappear in the integument until hatched.
The young Pipa Toads are rapidly developed in these dorsal cells, but they are extricated at a less advanced stage than almost any other vertebrate animal. After extrication, the tadpole grows rapidly, and the chief change of form is witnessed in the gills. As to the mother Batrachian, it is only after she has got rid of her progeny that she abandons her aquatic residence.[9]
The Batrachians differ essentially from all other orders of Reptilia. They have no ribs; their skin is naked, being without scales. The young, or tadpoles, when first hatched, breathe by means of gills, being at this stage quite unlike their parents. These gills, or branchiæ, disappear in the tailless Batrachians, as the Frogs and Toads, in which the tail disappears, are called. In the tadpoles the mouth is destitute of a tongue, this organ only making its appearance when the fore limbs are evolved. The habits also change. The tadpole no longer feeds on decomposing substances, and cannot live long immersed in water. The branchiæ disappear one after the other, by absorption, giving place to pulmonary vessels. The principal vascular arches are converted into the pulmonary artery, and the blood is diverted from the largest of the branchiæ to the lungs. In the meantime the respiratory cavity is formed, the communicating duct advances with the elongation of the œsophagus, and at the point of communication the larynx is ultimately developed. The lungs themselves extend as simple elongated sacs, slightly reticulated on the inner surface backwards into the abdominal cavity. These receptacles being formed, air passes into and expands the cavity, and respiration is commenced, the fore limbs are liberated from the branchial chambers, and the first transformation is accomplished.
The alleged venemous character of the Common Toad has been altogether rejected by many naturalists; but Dr. Davy found that venemous matter was really contained in follicles in the true skin, and chiefly about the head and shoulders, but also distributed generally over the body, and on the extremities in considerable quantities. Dr. Davy found it extremely acrid, but innocuous when introduced into the circulation. A chicken inoculated with it was unaffected, and Dr. Davy conjectures that this acrid liquid is the animal's defence against carnivorous Mammalia. A dog when urged to attack one will drop it from its mouth in a manner which leaves no doubt that it had felt the effects of the secretion.
In opposition to these opinions the story of a lad in France is told, who had thrust his slightly wounded hand into a hole, intending to seize a Lizard which he had seen enter. In place of the Lizard he brought out a large Toad. While holding the animal, it discharged a milky yellowish white fluid which introduced itself into the wound in his hand, and this poison occasioned his death; but then it is not stated that the boy was previously healthy.
Warm and temperate regions with abundant moisture are the localities favourable to all the Batrachians. Extreme cold, as well as dry heat, and all sudden changes are alike unfavourable to them. In temperate climates, where the winters are severe, they bury themselves under the earth, or in the mud at the bottom of pools and ponds, and there pass the season without air or food, till returning spring calls them forth.
The species of this family are very numerous. MM. Duméril and Bibron state that the Frogs, Rana, number fifty-one species; the Tree Frogs, Hyla, sixty-four; and the Toads, Bufo, thirty-five. They are found in all parts of the world, the smallest portion being found in Europe, and the largest in America. Oceania is chiefly supplied with the Tree Frogs. There are several curious forms in Australia, and one species only is known to inhabit New Zealand. The enormous fossil Labyrinthodon, of a remote geological era, is believed to have been nearly related to these comparatively very diminutive Batrachians.[10]